One of the most important operating characteristic values for controlling and/or regulating an internal combustion engine is the speed of the internal combustion engine. The speed value is picked up by means of corresponding transmitters and, if need be, prepared for the further processing by means of signal adaptation and amplifications. It is known to monitor the tachometer to determine whether it delivers an output signal or not. If an output signal is present it is used for the control and/or regulation, while a non-present output signal of the tachometer triggers corresponding erroneous functions.
It is now possible that the tachometer delivers an output signal, but that the same does not have any meaningful value. Such meaningless speed signals may be generated by electromagnetic stray effects, by defects of the tachometer, or by mechanical defects of for example, the rotating part of the internal combustion engine which is in operating connection with the tachometer. The disadvantage of the known monitoring devices of tachometer signals consists in that they do not recognize such meaningless signals as being erroneous, but feed them for further processing as correct speed signals. Therefore, in the known systems no plausibility monitoring of the tachometer signals is performed.